The potential energy curves of OH

The hypothesis that the a and ir orbitals are in a pure precession (Section 5.5) relationship to each other is supported by ab initio calculations (Czarny, et al., 1971), which find dominant 2po character for these orbitals in the A2E+ and 4n states at the internuclear distance, Rc, of the curve crossing. The pure precession value of the 4IIi/2 A2X+ interaction is 3-1/2a+/2 = 57 cm-1, in satisfactory agreement with the value of 47 cm-1 obtained from the observed linewidths. 

For SH, with a calculated spin-orbit interaction of 162 cm-1 between the A2E+ and 4II states (Wheeler, et al, 1997), the ratio between the interaction strengths for SH and OH is in good agreement with the ratio of the atomic spin-orbit constants of sulfur and oxygen, namely about 2.5. 

A similar mchanism explains the predissociation of the A2E+ state of the hydrogen halide cations by a 4n state. For HBr+, where the spin-orbit interaction strength is very large, the width of the v = 4 level is equal to 800 cm-1 (exp Penno, et al, 1998, calc. Banichevich, et al, 1992). For HI, the width is so large that the spectrum becomes continuous (Lefebvre-Brion, 1991). 

Sometimes a predissociation cannot be understood in such a simple singleconfiguration picture. An example is the predissociation of the b4Ej state (see Fig. 7.32). This predissociation is very weak but has been detected as a line broadening by a Doppler-free laser technique. The width of the upper state of the b4E -a4nu transition in absorption is 0.05 cm-1 for v = 5, N = 9, a factor of 25 smaller than the width in OD. As the oxygen atom spin-orbit constant is responsible for both of these predissociations, a direct mechanism for Oj seems improbable. The b4E state is most likely predissociated by 

The first matrix element is zero and the second is calculated to be 140 cm-1. Thus the overall electronic matrix element has a value of about 11cm-1, which is sufficient to partly explain the observed width. 

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Figure 7.31 The potential energy curves of OH. [Prom Bergeman, et al., (1981).] where...

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